Revert "Update to the newest buildout."

This reverts commit c7f2fece036403738c11fe1c61e5a3302f96de2b.

Needed to get the branch in shape for inclusion with master.
We'll hold back on using the latest buildout 2.0 bootstrap until
2.0 final is out.
Revert "Passing -t to bootstrap, needed until 2.0 final"

This reverts commit b759c25c5b2ad861cbe07f18e6f814639ce54cce.

Another pre-buildout2-final revert
Re-added 'python' and 'executable' keys to the mock buildout part
to better mimick buildout's behavior.
The 'python' part is required on pre-2.0 buildouts.